1. Field of the Invention
This invention concerns ultra-high resolution foil tension mask color cathode ray tubes, and more particularly, relates to an improved front assembly for such tubes which has a shadow mask with aperture configurations that provide for brightness uniformity and color purity throughout the picture area.
2. Definitions
The following definitions are essential to an understanding of the present invention:
As used herein, the term "shadow mask" is a component of a color cathode ray tube located in spaced adjacency to the faceplate, one having a plurality of apertures for the passage of the electron beams that excite phosphors deposited on the screen of the faceplate. The shadow mask, noted as having circular or near-circular apertures, "shadows" the triads of phosphor deposits so that the proper beam falls upon the assigned ones of the phosphor deposits. The shadow mask is also referred to as a "color selection electrode", or "parallax barrier." Shadow masks that may benefit from the invention include the foil mask secured to a suitable mask support under high tension, as well as the conventional curved mask with its associated curved faceplate, as designed for ultra-high resolution.
As used herein, "beamlet" means that portion of a light beam, or an electron beam, upon passing through a mask aperture. A "light beamlet" is formed by ultraviolet light rays that irradiate the shadow mask during screening. An "electron beamlet" is formed by any one of the three electron beams which have their origin in a three-beam electron gun located in the neck of the cathode ray tube envelope.
As used herein, the term "light image" is that area of the screening surface upon which a light beamlet falls. A "beam spot" is the area upon which an electron beamlet falls.
As used herein, the term "screening surface" refers to the screening surface of the faceplate which, in the manufacturing process, receives successive layers of screening fluids, comprising the grille and the phosphor deposits. The term "screen" refers to the inner surface of the faceplate following the deposition of the grille and the respective phosphor deposits that emit red, green and blue light when excited to luminescence by electron beamlets.
As used herein, the term "negative guard band" means a condition in which the beam spots are larger than the target phosphor deposits by a predetermined guard band area. In negative guardband screens, the margin of safety, or "guard band" that prevents color impurities, conventionally comprises a light-absorbing material called the grille.
As used herein, the term "clipping" refers to the reduction in the radial width of a beamlet in passing through a shadow mask aperture at an angle, and in which the edges of the aperture intercept the light rays in photoscreening, or the electron beams during tube operation. The amount of clipping is a function of the thickness of the shadow mask and the angle at which the light rays or electrons approach the aperture. The thicker the mask and the greater the angle, the greater the clipping.
3. Prior Art
The following examples are being submitted to the Patent and Trademark Office for evaluation as to possible relevance to the claimed subject matter. The examples are believed to be the closest of the art of which applicant is aware, but applicant makes no admission as to its relevance in fact, to its legal sufficiency, or to its priority in time, nor does applicant represent that no better art exists. U.S. Pat. No. 2,947,899 to Kaplan. A compensated aperture mask structure is disclosed having a plurality of apertures which are round at the axial aperture, but distorted into an elliptical configuration by radial foreshortening as a function of the distance of the apertures from the axial aperture. U.S. Pat. No. 4,139,797. A system is disclosed for increasing tolerance to radial registration errors between the electron beam landing areas and the phosphor elements due to shadow mask doming during operation of the tube. The geometry of the beam landing areas and the phosphor elements are characterized by having off the tube axis smaller ones of the phosphor elements and the mask apertures radially compressed relative to larger ones without a corresponding azimuthal compression. The radial compression increases with increasing radius such that the tolerances in the radial direction increase off axis without a corresponding increase in azimuthal tolerance. The result is said to be increased tolerance to the doming-induced registration errors between the phosphor elements and the beam landing areas.